In recent years, various solid oxide fuel devices comprising fuel cells capable of obtaining electrical power by generating electricity using fuel (hydrogen gas) and air, as well as auxiliary equipment for running such fuel cells, have been proposed as next-generation energy sources.
Japanese Unexamined Patent Application 11.7-307163 (Patent Document 1) sets forth a fuel cell device. In this fuel cell device, the electrical power generated is varied in response to load.
Here, referring to FIG. 15, we explain a power supply system utilizing a fuel cell. FIG. 15 shows an example of a conventional system for supplying electrical power to a residence using a fuel cell. In the system, electrical power consumed by a residence 200 is supplied by a fuel cell 202 and a grid power 204. Normally, the maximum power consumed by residences is larger than the maximum rated power generatable by the fuel cell 202. Therefore, even in the residence 200 utilizing the fuel cell 202, insufficiency is made up for by the grid power 204, and electrical power is supplied to the residence from the fuel cell 202 and the grid power 204. Furthermore, even in situations where the generating capacity of fuel cell 202 is below the maximum rated power for a residence, a portion of the electrical power consumed by the residence 200 is normally supplied from the grid power 204 in order to prevent a reverse current flow of the generated power to the current power 204.
The grid power 204 is power-fed from a transmission line to an electrical distribution panel inside the residence, and is purchased power. In other words, the total of the electrical power generated by the fuel cell 202 and the grid power 204 corresponds to the power consumed by the residence 200. The fuel cell 202 obtains a monitor signal from a power demand detector 206 of the electrical power purchased by the residence 200, and based on this, it varies the power generated by the fuel cell 202. That is, the fuel cell 202 determines a base current Ii, which the current that the fuel cell 202 should produce, based on the monitor signal obtained from the power demand detector 206, and controls the amount of fuel, etc. supplied to the fuel cell module 208 to enable production of this base current Ii. Also, the base current Ii is set at or below a value corresponding to the maximum rated power of fuel cell 202, regardless of the power consumed by the residence 200.
The fuel cell module 208 built into the fuel cell 202 generally has an extremely slow response, making it difficult to change its generated power to follow changes of power consumed by the residence 200. Therefore, a base current Ii signal which instructs to a fuel cell module 208 an amount of electrical power generated by fuel cell module 208 is determined by applying a filter 210 which performs integration or the like on the monitor signal, so that it changes extremely gradually compared to the change in power consumption.
The fuel cell 202 supplies the fuel cell module 208 with an amount of fuel proportional to the base current Ii so that the fuel cell module 208 has the capacity to produce the base current Ii. At the same time, an inverter 212 extracts a DC current Ic from the fuel cell module 208 and converts this to AC and supplies it to the residence 200. The actual extracted current Ic, which the inverter 212 extracts from the fuel cell module 208, is at all times set at or below the value of the base current Ii, and does not exceed the generating capacity of the fuel cell module 208. If a current equal to or greater than the generating capacity corresponding to the fuel supply amount, etc. determined based on the base current Ii is extracted from the fuel cell module 208, there is a risk that fuel depletion in fuel cells within the fuel cell module 208 will occur, causing dramatically shortening the life span of the fuel cells and damaging the fuel cells.
At the same time, because of sharp fluctuations in the power consumed by the residence 200, when consumed power suddenly drops, the residence 200 power consumption drops further than the power corresponding to the base current Ii, which is slowly varied.
In the fuel cell apparatus set forth in Japanese Unexamined Patent Application H07-307163, when the current value is increased to adapt to this type of delay in the fuel cell module 208, the set current value is updated through a delay setting instrument, and problems such as fuel depletion are prevented by delaying the increase in the set current value. Also, in this fuel cell apparatus, when updating the set current value, changes are always made in increments of subtracted current values or added current values. Therefore, the rate of change at which the said current value is changed is always fixed.
Japanese Unexamined Patent Application H7-307163